DPC - BS6515 vs Zedex "high performance"

[BTC Builder]

A bit of a strange one. My drawing says to use "high performance Zedex DPC or similar approved" for the all dpc/tray at dpc level.

 

I've never heard of this product and none of the local merchants stock it. 

 

When reading about it, it says that it does not conform to BS6515 standard which is what I'd associate with "normal" dpc rolls.

 

Anyone any experience with this stuff? I'm a bit wary of it tbh. I'm also installing a radon barrier which is perhaps why they've put this Zedex stuff on the drawing in addition to the radon barrier.

[nod]

It all does the same job 

Though it’s worth using 600 mil high load dpc on the GF to attach to the screed membrane 

[BTC Builder]

  On 04/04/2025 at 20:39, nod said:

It all does the same job 

Though it’s worth using 600 mil high load dpc on the GF to attach to the screed membrane 

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I'm already doing that with the radon barrier.

 

it's a detail I've never seen before, radon barrier across level from internal to external skin at dpc level, then a tray down from the first course internal to dpc external. Two trays if you will.

[ETC]

It’s a Visqueen product and is - I’ve been told - radon proof.

 

Easiest way to install a radon barrier is to:

Build up your sub-floor blockwork to the underside of your slab level.

Fill voids between rising walls and finish off with well compacted hardcore and 50mm of blinding.

Lay a 1200 gauge membrane (minimum gauge for a radon barrier) over the complete footprint of the works.

Overlap and tape all joints.

Use top-hats for services.

Set up a 150mm slab complete with anti-crack mesh and bear the slab 100mm onto the sub-floor blockwork.

Pour the slab.

Put a stepped DPC around the perimeter of the slab.

[nod]

  On 04/04/2025 at 20:53, ETC said:

It’s a Visqueen product and is - I’ve been told - radon proof.

 

Easiest way to install a radon barrier is to:

Build up your sub-floor blockwork to the underside of your slab level.

Fill voids between rising walls and finish off with well compacted hardcore and 50mm of blinding.

Lay a 1200 gauge membrane (minimum gauge for a radon barrier) over the complete footprint of the works.

Overlap and tape all joints.

Use top-hats for services.

Set up a 150mm slab complete with anti-crack mesh and bear the slab 100mm onto the sub-floor blockwork.

Pour the slab.

Put a stepped DPC around the perimeter of the slab.

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Isn’t google a wonderful thing 😁

[ETC]

You won’t find that specification on Google!

Edited April 4 by ETC

[BadgerBadger]

We used it for our DPC (no radon barrier) and it was good, notably thicker and stood up well on-site where we left sections to lap dpm onto later. That said it was harder to form shapes/bends etc so we used thinner stuff for cavity trays above windows etc. 

[Gus Potter]

  On 04/04/2025 at 20:23, BTC Builder said:

A bit of a strange one. My drawing says to use "high performance Zedex DPC or similar approved" for the all dpc/tray at dpc level.

 

I've never heard of this product and none of the local merchants stock it. 

 

When reading about it, it says that it does not conform to BS6515 standard which is what I'd associate with "normal" dpc rolls.

 

Anyone any experience with this stuff? I'm a bit wary of it tbh. I'm also installing a radon barrier which is perhaps why they've put this Zedex stuff on the drawing in addition to the radon barrier.

Expand  

@BTC Builder Well done you for spotting this and asking the question. Honestly I can't commend you enough. You have probably saved the SE's arse and most importantly made the folk / the public safe by asking this question.

 

This is a bit long winded.. take what I discuss below and make hay! I'm going to try in laymans terms to offer up an explanation.

 

For all, often when we design a masonry wall it often has downwards load that is a bit off the centre of gravity of the wall. There is sideways load from the wind. Sometimes these may be a masonry wall round a school playground.. big red flags! When we use plastic DPC's and membranes these break the small tensile strength in the wall that is required when we do the bending  / stability critical safety check. This bending force in the wall arises when we take wind, and off centre grqvity loadings into account.

 

Now many young SE's are under pressure in the work place. There is a small switch in most software that lets you select the type of DPC. A high bond Zedex is a highly specialised DPC that has not least a sanded surface top and bottom which replicates the way a mortar bed behaves. Thus in the software it appears as a continuous wall. Now technically I can design walls this way.. but first and foremost I would put a massive warning box on my drawings to say this is a critical element.. the contractor will often say to me.. Gus you are asking too much and I'm going to pass this high extra  cost (the extra labour / brickie skill level required) onto your Client and I'll leave it up to you to justify this added cost element. As a designer I am responsible for not just the SE sums but also the buildability.. as per new regs on Principle Designers etc..

 

@BTC Builder That is why you don't find this product as an off the shelf item any more. For you if you agree to install it you have to follow the instructions. It takes time and care.. and that will eat into your profit. If you have signed a contract to install and your bollocks are up to the post then you might want to find a way to do a deal. If I was say acting for you then I would look at the radon barrier position.. there is a chance the SE has shot themselves in the foot! The may have changed the setting in the software and forgotten about the radon barrier which breaks the bond in the wall.

 

In this day and age we avoid using DPC's that are able to act like the masonry bed as say Zedex.. it's just too risky. There are school walls falling down all over the place and compromised designs where SE's just don't have a clue about the costs on small self builds or extensions.

 

  On 04/04/2025 at 20:39, nod said:

It all does the same job

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Nod.. I know you have a pile of experience but this is something that is not bedtime reading.. you have to be a bit of an anal twat like me to pick up on this stuff.. but on the other hand we all need to realise that every day is a school day.. and the older we get the more we realise how much we don't know.  And I'm saying to you.. I admire what you do but you don't know it all as I .. and small things like this if missed can be dangerous! as in using high bond DPC's can be very dangerous.. I know a lot about this to be blunt and that is why I'll have to sing @BTC Builder builders praises again.

 

@ETC We get to a certain age where we are good enough at business to put the bread on the table. Then when we all put out heads together we have fun in designing particularly odd stuff which personally floats my boat, make a bit of profit, get to truck about and get invited socially into the things we have hand in desgining. Shite job eh?

 

 

 

 

 

 

[BTC Builder]

  On 04/04/2025 at 22:03, Gus Potter said:

@BTC Builder Well done you for spotting this and asking the question. Honestly I can't commend you enough. You have probably saved the SE's arse and most importantly made the folk / the public safe by asking this question.

 

This is a bit long winded.. take what I discuss below and make hay! I'm going to try in laymans terms to offer up an explanation.

 

For all, often when we design a masonry wall it often has downwards load that is a bit off the centre of gravity of the wall. There is sideways load from the wind. Sometimes these may be a masonry wall round a school playground.. big red flags! When we use plastic DPC's and membranes these break the small tensile strength in the wall that is required when we do the bending  / stability critical safety check. This bending force in the wall arises when we take wind, and off centre grqvity loadings into account.

 

Now many young SE's are under pressure in the work place. There is a small switch in most software that lets you select the type of DPC. A high bond Zedex is a highly specialised DPC that has not least a sanded surface top and bottom which replicates the way a mortar bed behaves. Thus in the software it appears as a continuous wall. Now technically I can design walls this way.. but first and foremost I would put a massive warning box on my drawings to say this is a critical element.. the contractor will often say to me.. Gus you are asking too much and I'm going to pass this high extra  cost (the extra labour / brickie skill level required) onto your Client and I'll leave it up to you to justify this added cost element. As a designer I am responsible for not just the SE sums but also the buildability.. as per new regs on Principle Designers etc..

 

@BTC Builder That is why you don't find this product as an off the shelf item any more. For you if you agree to install it you have to follow the instructions. It takes time and care.. and that will eat into your profit. If you have signed a contract to install and your bollocks are up to the post then you might want to find a way to do a deal. If I was say acting for you then I would look at the radon barrier position.. there is a chance the SE has shot themselves in the foot! The may have changed the setting in the software and forgotten about the radon barrier which breaks the bond in the wall.

 

In this day and age we avoid using DPC's that are able to act like the masonry bed as say Zedex.. it's just too risky. There are school walls falling down all over the place and compromised designs where SE's just don't have a clue about the costs on small self builds or extensions.

 

Nod.. I know you have a pile of experience but this is something that is not bedtime reading.. you have to be a bit of an anal twat like me to pick up on this stuff.. but on the other hand we all need to realise that every day is a school day.. and the older we get the more we realise how much we don't know.  And I'm saying to you.. I admire what you do but you don't know it all as I .. and small things like this if missed can be dangerous! as in using high bond DPC's can be very dangerous.. I know a lot about this to be blunt and that is why I'll have to sing @BTC Builder builders praises again.

 

@ETC We get to a certain age where we are good enough at business to put the bread on the table. Then when we all put out heads together we have fun in designing particularly odd stuff which personally floats my boat, make a bit of profit, get to truck about and get invited socially into the things we have hand in desgining. Shite job eh?

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So would the summary be, avoid and stick to polythene trays above the radon tray?

 

I noticed NHBC are now saying polythene trays should be avoided which probably contributed to the architect specifying zedex. 

 

Edited April 5 by BTC Builder

[Gus Potter]

  On 05/04/2025 at 20:28, BTC Builder said:

So would the summary be, avoid and stick to polythene trays above the radon tray?

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No,  you need to go back to the SE AND the Architect or the person who made the drawings and ask why there is a high bond DPC.

 

If you decide to swap DPC's then you are exposing yourself to a big risk. You could also be acting in a dangerous way. If BC come out and find you have swapped out materials then you could be in big trouble.

 

Re read my post. Be safe and ask the question.. it costs nothing. It's OK to ask "silly questions".. what is stupid is not to ask the question at all.

  On 05/04/2025 at 20:28, BTC Builder said:

I noticed NHBC are now saying polythene trays should be avoided which probably contributed to the architect specifying zedex.

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I think you are mixing up the different types of NHBC guidance. We used to make cavity trays etc out of DPC (folding it and so on to make a tray) and before that heavy duty bituminous felt. .. but modern brickies are crap at it.. at it so the NHBC are leaning towards factory made trays. The High bond Zedex is more associated with the structural performance of the walls and that is why I'm doing my best to encourage you to just ask question.